Objects often include layers of different compositions that are difficult to measure directly and accurately. In many cases, the object's interior can not be accessed to allow for direct measurement. It may be impractical to intrude the object's interior or, if even using non-invasive techniques, it may be difficult to position the probe for accurate measurements.
For measurements of biological specimens, the thickness of underlying layers are particularly inconvenient to measure. Many such measurements are preferably taken in vivo, which makes invasive techniques impractical. If non-invasive techniques are used, they are often susceptible to operator errors and can be quite costly, as in the case of expensive medical diagnostic equipment.
In the case of body fat measurements, skin calipers and water immersion tanks can be used to assess body fat. Such techniques have a number of drawbacks.
The principle behind the use of skinfold calipers is that the amount of subcutaneous fat correlates to percent body fat (American College of Sports Medicine, ACSM's guidelines for exercise testing and prescription, 53-63 (1995)). With a skinfold caliper measurement, after the skin is pinched by an operator without inducing pain to the subject, the thickness of the skinfold is measured with the caliper. Caliper measurements of skinfold thickness have been used with various equations developed to predict body density and percent body fat (American College of Sports Medicine, ACSM's guidelines for exercise testing and prescription, 53-63 (1995)). Most of these equations, however, are sex-specific or only apply to certain populations. Other equations to estimate body density and percent body fat have been developed using regression models that can take into account data from larger population based studies (Jackson, A. S., Pollock, M. L., Br J Nutr, 1978: 497-504 (1978)).
Even with these improvements, however, skinfold calipers are subject to several serious sources of errors. First, skinfold caliper measurements are heavily operator dependent. The force used to pull back the skin by the operator and the location of the measurement site may vary significantly between different operators, or the same operator, resulting in poor reproducibility of measurements. Second, even though skinfold caliper measurements are based on the assumption that subcutaneous fat thickness correlates to percent body fat, skinfold calipers cannot measure the thickness of subcutaneous fat directly. Skinfold caliper measurements, instead, provide an estimate of subcutaneous fat thickness which, in turn, is then used to estimate percent body fat. Thus, two approximations are used to estimate percent body fat. Third, skinfold caliper measurements may overestimate subcutaneous fat thickness. When the skinfold is pulled back for the measurement, fat from adjacent sites can be pulled toward the measurement site causing an artificial increase in the amount of subcutaneous fat present in the selected body region. This problem is exaggerated in subjects with very elastic soft-tissue. Fourth, the inaccuracies associated with skinfold caliper measurements have lead to the use of equations requiring measurements of 3 body sites, 4 body sites, and even 7 body sites (American College of Sports Medicine, ACSM's guidelines for exercise testing and prescription, 53-63 (1995)). However, even with these adjustments the inherent inaccuracies of skinfold caliper measurements, most importantly the inability to measure subcutaneous fat thickness directly, cannot be completely compensated.
Hydrostatic weighing is commonly considered the gold standard for determining body density and estimating percent body fat. Hydrostatic weighing relies on Archimedes' principle. A body submerged in water is buoyed by a counterforce equal to the weight of the water that it displaced. Bone and muscle tissue are denser than water, while fat tissue is less dense. Therefore, a person with low percent body fat will have higher body density and weighs more in water than a person with higher percent body fat and the same air weight. Conversely, a person with higher percent body fat for the same air weight will weigh less in water.
Although hydrostatic weighing is considered the gold standard for body fat determinations, it is subject to several sources of error. First, hydrostatic weighing requires estimation of pulmonary residual volume, which may vary significantly between individuals. Although pulmonary residual volume can be measured using pulmonary function tests, this adds extra time and expense to the procedure, which could decrease patient compliance. Second, hydrostatic weighing does not account for the variability in bone density known to exist between different individuals and races (American College of Sports Medicine, ACSM's guidelines for exercise testing and prescription, 53-63 (1995)). In patients with high bone density, hydrostatic weighing will underestimate percent body fat. Conversely, in osteoporotic patients, hydrostatic weighing may seriously overestimate percent body fat. Third, hydrostatic weighing requires large and expensive displacement chambers, and complete patient immersion in water. The technical requirements and the expense of hydrostatic weighing limit its use in frequent longitudinal measurements of percent body fat that are desirable in ambulatory patients undergoing diet or exercise induced fat reduction. Fourth, submersion of the head underwater may be difficult or anxiety provoking for some individuals.
Consequently, the present inventors have recognized the need to provide low cost and accurate ultrasound devices and methods for such applications, particularly hand held devices capable of being operated by untrained operators. Methods and devices are provided herein to provide for cost effective measurements and accurate of layer thickness, such as fat layer thickness.